A holey structure is a holey fiber (or fibre), also often called microstructure or photonic crystal fiber. Holey fiber is featured by an array of holes (hollow regions, capillaries) running along the fiber length. Some capillaries of this array may be filled with materials different from the material of the cladding thus forming one or more cores.
It is common to use holey fibers for guiding light. To this end, holey fibers need to have appropriate parameters of cladding and hollow regions (capillaries). The controlled parameters may include fiber and hollow regions sizes and hollow regions spacings. For example, holey fibers may be constructed to allow the light field to dominantly propagate in an air hole (for so-called air-guiding photonic bandgap fibers). Hence low-loss transmission of light through holey fiber can be achieved.
Various light guiding applications as well as probing applications utilizing light or other fields, require small-size devices to be applied to specific, very small locations on a sample (e.g. human body).